The invention relates to a pressurized water nuclear reactor vessel having support columns supporting an upper support plate above an upper core support plate in its upper plenum and, more particularly, to a pressure vessel having slotted support columns for guiding reactor coolant flowing from the upper core support plate into the upper plenum.
In the power operation of a pressurized water nuclear reactor facility, reactor coolant absorbs heat from fission reactions of pelletized fuel contained within thousands of one inch or smaller diameter fuel rods arranged in about 120 to about 190 fuel assemblies supported in the core of a reactor vessel by a lower core support plate and an upper core support plate. Hot coolant streams then flow from the core region upwardly through various passageways in the upper core support plate into an upper plenum where the several coolant streams mix together and then flow from the upper plenum through a hot leg to a steam generator.
In commercial facilities, fuel assemblies having different enrichments are loaded into the core in patterns which provide a uniform power distribution throughout the core. In low leakage fuel assembly loading patterns, the most highly enriched fuel assemblies are generally located between peripherally located lower enrichment fuel assemblies (which advantageously reduces the neutron fluence and improves the core efficiency) and the lowest enriched fuel assemblies in the central core region. However, calculations have shown that the temperatures of the coolant streams flowing through the passageways above the various regions of a core having a low leakage pattern may vary up to about thirty to fifty degrees Fahrenheit or more. The inventors have determined that these coolant streams may not sufficiently thermally mix together in the upper plenum above the upper core support plate and that this thermal condition may cause hot leg streaming of the coolant flowing through the hot leg with significant temperature deviations from the bulk coolant temperature. Hot leg streaming may result in inaccurate readings by hot leg resistance temperature detectors and uncertain heat balances around the reactor.